1. Field of the Invention
The present invention relates generally to a magnetic resonance (MR) imaging system, which employs nuclear magnetic resonance to display desired sectional images of the subject, and more particularly to an MR imaging system for use with MR fluoroscopy wherein high-rate imaging, reconstruction and display are sequentially performed to display realtime images sequentially.
2. Description of Related Art
An MR imaging system measures the distribution of intensity and relaxation time of nuclear spins at a desired examined part of the subject by employing nuclear magnetic resonance, and displays desired sectional images of the subject in accordance with the measurement data.
A publication xe2x80x9cMR Fluoroscopy: Technical Feasibilityxe2x80x9d (by S. J. Riederer, T. Tasciyan, F. Farzaneh, J. N. Lee, R. C. Wright, R. J. Herfkens, Magnetic Resonance in Medicine 8, 1988, pp. 1-15) introduced MR fluoroscopy as an imaging method using the MR imaging system. In the MR fluoroscopy, high-rate imaging, reconstruction and display are performed sequentially, periodically and continuously for a slice of the subject, and realtime images of the slice are displayed. The image reconstructing time for the slice is substantially adjusted to the imaging time for the slice so that the images of the slice can be displayed in realtime. The MR fluoroscopy is used to monitor and control interventional radiology (IVR), wherein the operator practices thermotherapy, examines, and/or treats the subject while performing the MR imaging to the subject. In the case of the IVR, the MR fluoroscopy enables the operator to determine the position and advancing direction of an insertion (e.g. a needle for puncture or biopsy, and a catheter) in realtime while the operator moves the needle towards a target (e.g. an affected part) in the subject. The puncture is usually performed in biopsy, injection and laser surgery for the affected part.
In the conventional MR fluoroscopy, however, only a specific slice is imaged. Hence, if the needle is displaced from the specific slice while the operator moves the needle toward the target in the subject, the shade of the needle disappears from the displayed image of the slice. This makes it impossible for the operator to determine the position and advancing direction of the needle.
Accordingly, it is an object of the present invention to provide an MR imaging system that always enables the operator to correctly determine the position and advancing direction of the insertion.
To achieve the above object, the present invention is directed to an MR imaging system comprising: a static magnetic field generator for generating a static magnetic field; a magnetic field generator for applying a radio frequency magnetic field and a gradient magnetic field to a subject placed within the static magnetic field; a signal detector for sequentially detecting nuclear magnetic resonance signals produced from a plurality of slices of the subject; a signal processor for sequentially reconstructing a plurality of images of the slices from the detected nuclear magnetic resonance signals; and an image display system for showing the plurality of reconstructed images.
One of the slices may include an insertion that is inserted into the subject, and it may be parallel with a direction in which the insertion should advance.
The slices may be parallel or perpendicular to each other, and they may intersect each other at such an angle as to form a stereoscopic image so that they form a stereoscopic pair.
A pulse sequence for imaging (an imaging pulse sequence) is not particularly restricted if signals required for reconstructing one image can be detected within a short period of time like a gradient echo method and an echo planer method. The signal detection time is controlled by adjusting the number of phase encoding and a repetition time TR.
A multi-slice imaging may be employed to reconstruct images of slices. In the multi-slice imaging, the excitation and the signal detection for slices are sequentially performed within a repetition time, and this is repeated a plurality to acquire signal required for reconstructing respective images of the slices. It is also possible to reconstruct images of slices by repeating a single-slice imaging, which performs the excitation, signal detection and image reconstruction for a single slice, a plurality.
According to the present invention, not only one sectional image but an image of a slice that intersects the sectional image or is parallel thereto in the vicinity thereof are reconstructed and displayed substantially at the same time. If the insertion is displaced from one slice and a shade of the insertion disappears from the sequentially-displayed image of the slice while the operator moves the insertion towards an affected part in the subject, the shade of the insertion appears on the image of another slice that intersects the sectional image or is parallel thereto in the vicinity thereof. Hence, the operator can always determine the position and advancing direction of the insertion, and can perform the IVR easily and accurately.
Particularly since a stereoscopic image is constructed by images of two slices that intersect one another at a predetermined angle, or images of slices are combined and displayed three-dimensionally, the position and advancing direction of the insertion can be displayed more clearly.